Beginning with the invention of the bipolar transistor, modern technology has increasingly sought devices that are both smaller and faster than are prior art devices. One important development in the search for faster devices was the field effect transistor (FET). The first realizable FET was the junction field effect transistor (JFET) which was first analyzed in detail by Shockley in 1952. A JFET is essentially a voltage-controlled resistor which has many attractive features that make its use desirable in applications such as, for example, switching, amplifiers, and integrated circuits. Another type of FET is the metal-oxide-semiconductor field effect transistor (MOSFET) which is currently the most important device for integrated circuits used in applications such as memories. Field effect transistors are unipolar devices, that is, their current conduction primarily involves only one type of carrier, and do not suffer from minority carriage storage effects as do bipolar transistors. Field effect transistors therefore should have higher speeds and cut-off frequencies than do bipolar devices.
Above the threshold, a MOSFET is basically a field effect device. The field effect arises from the screening of the electric field under the gate electrode by either an accumulation or depletion of mobile charges in the channel. However, in its subthreshold regime, the drain current in a MOSFET is due to thermionic emission from the source which plays a role that is analogous to that of a cathode. The potential barrier between the source and the channel decreases linearly with the band bending which is controlled by the gate voltage. In this regime, i.e., subthreshold, the MOSFET may be viewed as a potential effect rather than a field effect device.
Device operation in the subthreshold regime, rather than the threshold regime, would have several desirable attributes. The transconductance increases with the gate voltage, V.sub.G, only in the subthreshold regime. That is, an increase in V.sub.g beyond threshold does not yield an advantage in speed since both the output current, I, and the capacitively stored charge, Q, are, above threshold, proportional to the gate voltage. The response time, .tau., which is proportional to Q/I, therefore, does not improve further.
However, while the charge injection, i.e., potential effect, mode of operation of FETs is theoretically more attractive than the field effect mode, the latter mode is preferred in practice. One reason for this preference is the fact that high values of output current cannot be achieved in the subthreshold regime. For an oxide thickness of approximately 500 Angstroms and a saturated velocity of approximately 10.sup.7 cm/sec, the maximum current that a device can produce is typically of the order of 10.sup.-2 A/cm of gate width. The subthreshold current is thus generally insufficient for fast device operation because of the parasitic capacitances.
There is also a second reason why FETs are generally not operated in their subthreshold regime. This reason is due to the uncertainty in the threshold voltage that necessarily results from variations in processing. The voltage swing must be reduced to several kT/q for a sufficiently small uncertainty and unfortunately, the reproducibility of device parameters, such as the surface condition, makes it difficult, if not impossible, with current technology, to control the height and shape of the potential barrier.
Several structures have been proposed in which the height of the barrier is controlled by an externally applied voltage. For example, IEEE Transactions on Electron Devices, ED-27, pp. 1128-1141, June 1980, proposes a structure that is termed a permeable-base transistor. The ideal structure proposed has a thin metal grating embedded in a single semiconductor crystal. The metal grating forms a base contact and is positioned in the semiconductor crystal between the collector and emitter contacts. The metal grating forms a Schottky barrier with the semiconductor and controls the flow of electrons from the emitter to collector. This device constituted an improvement on what is commonly termed, by those in the art, the static induction transistor. This type of transistor was described in IEEE Transactions on Electron Devices, ED-22, pp. 185-197, April 1975. Controlling electrodes in this transistor structure were formed by highly doped regions which were complementary to the source.
While these devices may be advantageously employed in some applications, they are not ideal devices for all applications for several reasons. For example, the permeable-base transistor faces practical problems in its implementation as high quality semiconductor material must be grown over the metallic grid and in practice, this is often difficult.